The present invention is directed to improvements in methods for locating faults in underground, unshielded, insulated electrical cables by establishing a voltage gradient through the earth emanating from the fault, and sensing the direction of the gradient to locate the fault.
Insulated electrical cables buried in the earth often develop faults due to insulation failure. If the faulted cable has an aluminum conductor, the fault area tends to develop a high-impedance aluminum hydroxide coating due to exposure to moisture in the ground; alternatively, if the faulted cable has a copper conductor, a high-impedance copper sulfate coating often develops at the fault.
A common method of locating the fault has been the so-called earth gradient method. The faulted cable is isolated at both ends and then a pulsed voltage is applied between one end of the faulted cable and a ground rod. When the pulsed voltage overcomes the impedance of the fault (i.e. "flashes" the fault), fault current travels from the fault to the ground rod. The fault current sets up a voltage gradient field in the earth whose direction can be detected, usually by a high-gain amplifier feeding a "zero" center meter. The inputs to the detector are two probes that can be pushed into the ground to sense the voltage gradient present. The detector is moved down the route of the faulted cable and, at regular intervals, the input probes are imbedded in the ground at positions spaced from each other. The input probe that is closest to the fault will deflect the meter toward that probe. If the detector passes beyond the fault, the meter deflects in the opposite direction, because the opposite input probe is now closer to the fault. When the two probes are at an equal distance on each side of the fault, the meter will not deflect.
Because of the expected high impedance at the fault location, the conventional practice has been to apply the pulsed voltage initially to the faulted cable at a high level at or near the maximum voltage of which the voltage source is capable, for example at several thousand volts. After the gradient has been established (i.e. after the fault has been flashed), the impedance decreases and the voltage automatically reduces as current increases, due to the output power limitations of the voltage source. A current meter connected to the ground rod is normally used to detect the establishment of the voltage gradient. If the fault impedance should increase for any reason after initial establishment of the voltage gradient, the output voltage is likewise permitted to rise accordingly, even to the maximum voltage limit of the source if necessary.
The principal problem with the foregoing conventional practice is that the cable is often overstressed by being subjected to a voltage much higher than that for which it was designed, and much higher than is needed to flash the fault. The initial application of an excessively high voltage, even if only for a few pulses, can severely damage a cable. If such a high voltage is permitted to be reapplied after initial establishment of an earth gradient field, the risk of damage increases.